The aim of this study was to investigate the relationship between amniotic fluid insulin (AF-insulin) measurements and maternal blood glucose levels in pregnancies complicated by insulin-dependent maternal diabetes mellitus (IDDM). Twenty-five patients with IDDM underwent amniocentesis (AC) in the third trimester. Twelve patients had a second amniocentesis after 2-3 weeks. The maternal blood glucose values (MBG) 2 weeks before amniocentesis were correlated with AF-insulin. Mean (+/-S.D.) MBG in the group with AF-insulin > 97th centile (n = 7) was 6.1 +/- 1 mmol/l. MBG in the group with AF-insulin < 97th centile (n = 18) was 5.3 +/- 1.2 mmol/l (r = 0.2948; P-value 0.162). In the group with repeated AC and AF-insulin > 97th centile (n = 6) the correlation coefficient was 0.722 (P = 0.043), whereas in the group with normal AF-insulin (n = 6) no correlation was found (r = -0.213; P = 0.686). These results indicate that no significant correlation exists between MBG values and concentration of AF-insulin. MBG is not appropriate for the diagnosis of fetal hyperinsulinism in well-controlled women with IDDM. In individual cases with AF-insulin > 97th centile a decrease of MBG causes lower AF-insulin levels. These results indicate that there seems to be an individual threshold for maternal MBG which causes hyperinsulinism. Fetal hyperinsulinism not only depends on blood glucose levels. Different fetal sensitivity to maternal glucose stimuli or a different glucose transport across the placenta in the individual fetus could be responsible for these results.